Monomode optical fibre lasers have a doped, active core of very small dimensions, typically, a few microns. The fact that the core only supports one mode because of its small size, is a major benefit and is a necessary feature in most applications of optical fibre lasers and amplifiers. Unfortunately, however, the small core dimension also means that the optical fibre laser can only be pumped effectively by a laser whose beam has essentially perfect spatial properties, that is, a single mode beam.
Diode lasers offer a convenient pump source but the power available as a single mode beam from diode lasers is limited. Much higher power, up to several watts, can be obtained from diode lasers in the form of arrays. However, in such arrays, the beam produced is highly multimode and not suitable for launching directly into the monomode fibre core.
One proposal for dealing with this difficulty is to use cladding pumping. The active doped monomode core is surrounded by a much larger cladding, which is undoped and, hence, transparent. This cladding is then surrounded by an outer cladding so that any light launched into the inner cladding is guided within the inner cladding in the same way that light is guided in a conventional optical fibre. This arrangement permits a multimode beam, from, say, a diode laser array, to be launched into the inner cladding and then, as it propagates, light from the multimode beam will leak into the monomode core. This enables the core to be pumped by a multimode beam, and eventually all of the absorbed multimode power will be converted into a monomode laser emission within the core. Thus, this arrangement can provide the major benefit of increasing the brightness of a high-powered diode array beam and can, consequently, be used in a large number of applications.
In accordance with a first aspect of the invention, there is provided an optical fibre laser having an optical fibre comprising a core; an inner cladding surrounding the core and an outer cladding surrounding the inner cladding and the core; and a light source, characterised in that the core is doped with a first laser active material, the inner cladding is doped with a second different laser active material and the light source is a multimode source arranged to pump the inner cladding so as to cause laser emission therein which, in turn, serves as pump radiation for the laser active dopant in the optical fibre core.
In accordance with a further aspect of the invention, there is provided an optical fibre for use in an optical fibre laser, the fibre comprising a core; an inner cladding surrounding the core, and an outer cladding surrounding the inner cladding and core, the fibre being characterised in that the core is doped with a first laser active material and the inner cladding is doped with a second, different laser active material such that, when the inner cladding is pumped so as to cause laser emission therein, the laser emission in the inner cladding, in turn, serves as pump radiation for the laser active dopant in the optical fibre core.
Thus, the inner cladding can be pumped by means of a diode array or any other appropriate multi-mode pump beam. The ends of the fibre are terminated in mirrors, in the usual optical fibre laser configuration, so that lasing occurs in the multimode inner cladding. Laser light produced in the inner cladding then leaks into the monomode core, where it serves as the pump radiation for the laser-active dopant in the core. This in turn lases, using the same end mirrors which are designed to have high reflectivity at the laser wavelength appropriate to the inner cladding dopant and the core dopant.
This arrangement has a number of benefits over the earlier cladding pumping arrangement utilising a transparent inner cladding.
Firstly, it permits the pump radiation from the diode array to be absorbed in a much shorter length since it is absorbed directly by the inner cladding rather than by leakage into the core. This shorter length has several advantages, for example, it allows short pulse operation when Q-switched and it avoids excessive losses. Secondly, the laser intensity in the lasing inner cladding can be much higher than the intensity of the light from the pump source. The optical fibre laser of the invention is, consequently, particularly well suited to pumping a 3-level laser material in the core. 3-level laser materials need higher pump intensities than 4-level lasers. Since many of the more potentially useful fibre lasers use three-level materials, this latter advantage is of great importance.
Whilst the benefits outlined above are the most important, there are other ways in which the use of the optical fibre laser of the invention could be beneficial. For example, incoherent lamps could be used to pump the inner core transversely, the inner core would then transfer power through multimode laser oscillation into the monomode core. Monomode output has reasonable efficiency using an incoherent lamp as pump source should, thus, be achievable.